<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .Grid {
            display: flex;
        }

        .Grid-cell {
            flex: 1;
            border: 1px solid red;
        }

        .Grid-cell.u-full {
            flex: 0 0 100%;
        }

        .Grid-cell.u-1of2 {
            flex: 0 0 50%;
        }

        .Grid-cell.u-1of3 {
            flex: 0 0 33.3333%;
        }

        .Grid-cell.u-1of4 {
            flex: 0 0 25%;
        }
    </style>
</head>

<body>
    <div class="Grid">
        <div class="Grid-cell u-1of4">...</div>
        <div class="Grid-cell">...</div>
        <div class="Grid-cell u-1of3">...</div>
    </div>
    <div class="Grid">
        <div class="Grid-cell u-1of2">...</div>
    </div>
</body>

</html>